The present invention relates to an x-ray tube and is particularly related to an apparatus for reducing the likelihood of electrical discharge between an x-ray tube envelope and an x-ray tube housing. Principles of the present invention find particular application in a corona shield integrally formed with weld members that join segments of the x-ray tube envelope. Features and principles of the present invention will be described with particular respect thereto.
Typically, a rotating anode x-ray tube includes an evacuated envelope comprised of glass which encloses a cathode assembly, a rotating anode assembly and a bearing assembly to facilitate anode rotation. An induction motor is provided to drive rotation of the anode. The induction motor includes a stator located external the evacuated envelope and a rotor attached to the anode assembly located within the envelope. Energizing the stator coils causes the rotor of the induction motor to rotate the anode in the bearing assembly.
Some higher power x-ray tubes, such as those used in Computed Tomography applications, have different portions of the evacuated envelope made of materials other than glass or in combination with glass. In some of these multiple material envelope x-ray tubes, the central portion of the envelope surrounding a rotating anode target is comprised of metal. The cathode end and anode end of the evacuated envelope is comprised of an insulator material such as a ceramic or glass.
Another common construction of multiple material x-ray tube envelopes is a single insulator portion joined with the metal envelope portion. The metal portion of the envelope extends from the tube center to one end of the x-ray tube. In this configuration the other end of the x-ray tube is enclosed by the insulator portion. For example, the metal envelope extends from the center of the tube to the anode end of the tube and the insulator portion surrounds the cathode end of the x-ray tube. In this configuration, the anode can be kept at the same potential as the surrounding metal portion of the evacuated envelope.
The x-ray tube and induction motor is enclosed in a housing assembly which is used to mount the x-ray tube in an imaging system as well as provide for cooling and electrical connections for operation of the x-ray tube. The housing contains a fluid, such as a dielectric electrical insulating oil having high electrical resistance, to provide electrical insulation for the high voltage connections. The high-dielectric strength oil is a very effective insulating medium for filling interstitial spaces between the components of the x-ray tube system as well as impregnating any porous and permeable materials within the components. In addition, the fluid is circulated through the housing and an associated cooling system to provide cooling for the x-ray tube. The x-ray tube housing is usually at ground potential.
During production of x-rays a current is passed through a cathode filament located in the cathode assembly. This current heats the cathode filament such that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high electrical potential, on the order of 75-200 kV, is applied across the cathode assembly and the anode assembly. The high voltage potential accelerates the thermionically emitted electrons and causes them to flow in an electron beam from the cathode assembly to the anode assembly. A cathode cup focuses the flowing electrons onto a small area, or focal spot, on a target of the anode assembly thereby generating x-rays. A portion of the generated x-rays pass through x-ray transmissive windows of the envelope and the x-ray tube housing.
Substantial heat is produced by the electron beam striking the anode during the generation of x-rays. The electrical insulating oil within the housing and surrounding the x-ray tube removes heat produced during the generation of x-rays. The properties, and useful life expectancy, of electrical insulating oils is affected by operating conditions of the x-ray tube.
Electrical insulating oils are typically characterized by two properties: Corona Inception Voltage (CIV) and dielectric strength. Corona is a luminous discharge attributed to ionization of the media surrounding a conductor or tube component having a high voltage. Corona can reduce the dielectric life time and ultimately cause dielectric failure of the insulating oil. High current densities associated with corona result in gasification of the dielectric medium, which in turn decreases the voltage level at which corona or ionization damage begins to occur; e.g., the CIV. Above the CIV, corona is intensified and a decrease in the insulating properties and useful life of the dielectric medium is seen. Below the CIV, corona still occurs, but at a much reduced level. In addition, corona in power components or systems increases exponentially as dielectric strength decreases. At some point, dielectric breakdown, an electrical short circuit through the oil, occurs as a result of corona.
Most of the corona by-products are gases that follow the laws of solution. The gasses form bubbles and reabsorb depending on the temperature and pressure under which the insulating oil is used. When the solution is near saturation, the gaseous contaminates are easily ionized by an electric field. Consequently, corona activity in electrically stressed oil increases over time. As the levels of the ionization products increase in the oil, the likelihood of arcing and tube failure can increase.
Both the CIV and the dielectric strength are significantly reduced by the presence of any contamination in the oil. Contamination, whether it be gaseous, moisture, or particulate, increases as oil ages, directly causes degradation of the insulating system, and ultimately can cause arcing as well as system or component failure. Several mechanisms, including corona, oxidation, heat, electrical stress, and moisture, are known causes of oil degradation and contamination build-up. Electrically stressing a component or system will cause corona or ionization of the insulating oil to occur.
In addition to breakdown in the oil resulting in greater likelihood of corona discharge and arcing, the shapes of surfaces of the x-ray tube envelope components can affect corona production and arcing. In the higher power multi material envelope x-ray tubes, the various metal and insulator evacuated envelope components have attached weld flanges made of electrically conductive metal. The weld flanges typically join the insulator portion and metal section of the envelope such that long thin sections of metal extend around the envelope and away from the tube envelope. The weld flanges are used to join adjacent envelope sections. The joined weld flanges result in surfaces that have abrupt edges. The edges result in a non-uniform electric field having irregular and substantially higher local electric field strength at the edge. These non-uniform higher electric field irregularities result greater likelihood of corona discharge, oil breakdown and arcing between the tube envelope and housing.
In addition, as an x-ray tube experiences normal operation in the field, the cooling fluid in the housing surrounding the envelope is exposed to high temperatures which breaks down the oil. When this heat related break down of the oil occurs, the dielectric properties of the oil are also adversely affected. This results in reduced dielectric strength of the electrically insulating oil and less electrical insulation between the high voltage components of the x-ray tube as well as the housing.
An arc is an undesired surge of electrical current between two elements of the x-ray tube system which are at a different electrical potential. In x-ray tubes, this tendency to arc often increases as the tube ages due to factors such as degradation of dielectric electrical insulating and cooling fluid within the housing surrounding the evacuated envelope. As the electrical insulating properties of the fluid decreases, the likelihood of arcing between the housing and the x-ray tube increases.
Arcing in an x-ray tube used in a Computed Tomography (CT) imaging system can contaminate the signal collected at the detectors and affects proper image reconstruction. This may result in an un-usable set of data requiring another CT scan of the patient.
Arcing typically occurs in the area of the x-ray tube having the highest electric field strength. As such, arcing in an x-ray tube may commonly occur at components or component interfaces which form edges or other structural features that cause increased localized electric field stresses when the component is at a high electric potential during x-ray tube operation.
The present invention is directed to an evacuated envelope weld member that satisfies the need to provide a junction between evacuated envelope components at high voltage x-ray tube operating potential which reduces corona discharge, arcing and breakdown of electrical insulating oil in x-ray tube systems. An apparatus in accordance with one embodiment of the present invention includes an x-ray tube comprising a first electrode and a second electrode. The first and second electrodes are located in operative relationship with one another to generate x-rays when the electrodes are energized at their respective operating potential. An evacuated envelope encloses the first and second electrodes. The evacuated envelope includes a first envelope wall portion, a second envelope wall portion and an envelope weld member comprising an electrical conductor. The envelope weld member is in electrical communication so as to be at operating potential of one of the first and second electrodes when the x-ray tube is energized. The envelope weld member is adapted for vacuum tight joining to the first envelope wall portion and to the second envelope wall portion. The envelope weld member has an integral corona shield portion.